Defining shear zone boundaries using fabric intensity gradients: An example from the east-central Nepal Himalaya

Abstract

Uranium-lead (U-Pb) zircon geochronology, whole-rock geochemistry, and petrographic observations indicate that specimens from a suite of variably deformed granite and orthogneiss from the Okhaldungha region of east-central Nepal share a common origin. Microtextural characterization and quartz crystallographic fabric preferred orientation analyses of these same specimens outline a strain gradient that marks the location of a shear zone boundary. The location of this boundary, at the base of the orthogneiss, coincides with one of the interpreted locations of the Main Central thrust, though it cannot be uniquely identified as such. This study provides the first steps toward empirical constraints on the location and geometry of thrust structures in the region, helping to clarify the complex local kinematic framework. These methods not only help in assessing orogenic models of the Himalaya, but may also be applied to investigating other orogenic systems where the potential location of shear structures is contested.